The star 18 Scorpii (HD 146233, HR 6060) is currently regarded as the nearest solar twin. We have been observing this star on our solar-stellar spectrograph program since 1995. In this Letter we present our Ca K observations of 18 Sco through 2000 July. The data show a pronounced rise that suggests 18 Sco has a well-defined activity cycle that reached an apparent minimum in 1998 and showed a rapid rise through the most recent of our year 2000 data. Comparison with contemporaneous solar data, obtained with the same instrument, suggests that the activity cycle of 18 Sco may be of greater amplitude than the Sun's and that its overall chromospheric activity level is noticeably greater than the Sun's. This excellent solar photometric twin therefore may be a less perfect spectroscopic twin.